Image formation in, for example, electrophotography, electrostatic recording and electrostatic printing is performed in accordance with a series of steps: forming a latent electrostatic image on a latent electrostatic image bearing member (hereinafter may be referred to as a “photoconductor” or an “electrophotographic photoconductor”); developing the latent electrostatic image with a developer to form a visible image (toner image); transferring the visible image onto a recording medium such as paper; and fixing the transferred image onto the recording medium to form a fixed image.
The developer is mainly classified into one-component developers containing only a magnetic or non-magnetic toner and two-component developers containing a toner and a carrier.
In general, from the viewpoint of achieving desired energy efficiency, image fixation in electrophotography is widely performed with a heating roller method in which a toner image on a recording medium is fixed by directly pressing a heating roller thereagainst. The heating roller method requires a large amount of electric power for performing image fixation. In view of this, various attempts have been made to reduce electric power consumed for a heating roller from the viewpoint of energy saving. For example, there is often employed a method in which when no image is output, the power of a heater for a heating roller is set to a low level; and when an image is output, the power is increased to raise the temperature of the heating roller.
However, in this method, it takes about several tens of seconds (waiting time) to raise the temperature of a heating roller at a sleep mode to a temperature required for image fixing, which is inconvenient for users. Also, in another desired method for reduction of electric power consumption, a heater is completely off when no image is output. In order to attain energy saving based on these method, it is required that the fixing temperature of a toner itself be lowered to decrease the toner fixing temperature in use.
In accordance with development in electrophotographic technology, toners used in developers have been required to be excellent in low-temperature fixing property and storage stability (blocking resistance). As a result, attempts have been made to use polyester resins instead of styrene-based resins conventionally used for binder resins of toners, since polyester resins have a higher affinity to, for example, recording media, and have a better low-temperature fixing property than styrene-based resins. For example, there have been proposed a toner containing a linear polyester resin whose physical properties (e.g., molecular weight) have been defined at predetermined values (see Patent Literature 1), and a toner containing a non-linear, cross-linked polyester resin formed by using rosin as an acid component (see Patent Literature 2).
In an attempt to further improve image forming apparatuses in processing speed and energy saving, conventionally used binder resins for toners are not still sufficient to meet the recent market requirements, making it very difficult to shorten the required fixing time in a fixing step and to attain a sufficient fixation strength when using a fixing unit whose temperature has been lowered.
As disclosed in Patent Literature 2, the toner containing a polyester resin formed by using rosin is advantageously excellent in low-temperature fixing property. In addition, it is readily pulverized to enhance toner productivity in the pulverization method, which is advantageous. Meanwhile, when 1,2-propanediol (a branched alcohol having 3 carbon atoms) is used as an alcohol component, the formed toner has a better low-temperature fixing property, while maintaining offset resistance, than that formed by using an alcohol having 2 or less carbon atoms. In addition, such an alcohol is effectively used for preventing degradation of storage stability of the toner caused by decrease in glass transition temperature thereof, as compared with the case where a branched alcohol having 4 or more carbon atoms is used. When the polyester resins formed from rosin and/or the above alcohols are used for a binder resin of toner, the formed toner is advantageous in that it is fixed at low temperature and improved in storage stability.
Meanwhile, demand for energy saving is expected to be more and more strict in future. At present, use of polyester resin excellent in low-temperature fixing property is gradually improving toners in low-temperature fixing property more than before. But, when such a polyester resin is only used; i.e., unless some additional measures are taken, it is difficult to sufficiently meet requirements for energy saving in near future.
In recent years, toners have been improved in low-temperature fixing property by adding a fixing aid thereto (see Patent Literature 3). Patent Literature 3 proposed that the fixing aid is made to exist in toner as crystal domains to improve it in both heat resistance/storage stability and low-temperature fixing property. But, in accordance with the recent development in high-speed image forming apparatuses, toners have been required to have high durability and meet requirements for further energy saving. At present, difficulty is encountered in sufficiently meeting the aforementioned requirements and thus, demand has arisen for further improvement and development.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2004-245854    Patent Literature 2: JP-A No. 04-70765    Patent Literature 3: JP-A No. 2006-208609